WiFi-Hal: De-couple roaming feature with Gscan

Per WiFi HAL design, roaming feature should have nothing to do with
Gscan, but in current impl, these two are tightly coupled:
- Roaming capabilities is queried by vendor cmd GSCAN_GET_CAPABILITY.
- Set blacklist bssid returns error if Gscan not supported

So de-couple roaming feature with Gscan by:
- Set default roaming capabilities if Gscan not supported.
- Set blacklist bssid returns error if Roaming not supported.

CRs-Fixed: 2405118
Change-Id: I9f96546538dd4d07a1f930f107c0024436907927